Encoders are often used in environments where they are subjected to significant vibrations, and often in a heavily contaminated environment. The encoder may, for instance, be used to generate the position values of shaft rotation in a machine. For instance, the encoder may be attached to a rotating shaft in the machine. The encoding disk is caused to rotate as the shaft rotates, and the encoder casing and detector are firmly affixed to the casing of the machine.
In certain conventional systems, an optoelectrical state encoder is used, which includes an encoding disk that has an optically readable pattern. The disk pattern is read by one or more sensor heads having at least one detector, which each deliver an electric signal in relation to the amount of light that is received in the detector, so that movement of the encoding disk in relation to the detector will be indicated by changes in the electric signal.
The encoding disk includes a periodic pattern, such as a plurality of reflective and non-reflective fields of the same size, for instance. When the encoding disk is turned or rotated, the change between reflective and non-reflective fields can be detected and a change in angle thus determined.
Besides the use of optical/reflective encoders, there are also encoders for magnetic, magnetoresistive and inductive scanning.
However, when the shafts have very large diameters, such conventional encoders meet their limitation due to the need for specialized mounting, such as toothed belt arrangements, that are complicated and expensive. Furthermore, conventional encoders cannot be used for shaft ends that are inaccessible or for hollow shafts. Recently, magnetic encoders have been introduced in the market, including a magnetic tape that can be installed on large shafts more easily and inexpensively compared to the conventional encoders. When a magnetic tape is used, a carrier ring is suitably installed on the shaft, which carrier ring supports the magnetic tape. A sensor head for reading the magnetic tape is arranged with a small air gap close to the magnetic tape on the carrier ring that is arranged on the shaft. In similar manner, when a sensor head is used for magnetoresistive, optical/reflective and inductive scanning, the gap between the sensor head and the shaft is arranged with a small air gap.
A problem with the use of magnetic, magnetoresistive, optical/reflective and inductive encoders is that the side of the sensor head housing facing the shaft or the carrier ring has to match the curvature of the shaft, as well as any carrier ring on the shaft. This is due to the fact that the detector(s) inside the housing of the sensor head should be arranged as close as possible and with the same distance to the periphery of the shaft, since the reading capability of the detectors decreases with increasing distance. Hence, the side of the sensor head facing the shaft or carrier ring should have the same curvature as the shaft or carrier ring when dual detectors are used. At least two detectors are necessary when using, for example, magnetic tape encoders in order to be able to sense the tape without interruptions over the joint on the tape. For practical reasons, a conventional sensor head is provided with dual detectors within its housing. Consequently, for any given shaft diameter, a respective housing of a sensor head is conventionally manufactured with a front wall, facing the shaft/ring, having a given curvature for the specific shaft dimension. Further, in such a conventionally manufactured sensor head, the detectors are fixed in an exact arrangement and other components are also arranged within the housing of the sensor head for the specific shaft dimension. To sum up, all the above mentioned requirements for the sensor head result in a sensor head that is expensive and complicated.
PCT International Published Patent Application WO 2011/018330 describes a pole wheel arrangement for a rotary encoder, in which the pole wheel includes pole wheel segments. The encoder has a sensor head with several sensors.